Rocker arms, valve guide sleeves, and other high wear parts for an engine have typically been made of solid material and in the case of rocker arms have usually been castings comprised of cast iron. These iron parts have required costly post-treatment, such as furnace heating, to improve wear characteristics. In the hope of reducing cost and weight, other materials have been substituted for the solid castings, such as sheet metal stampings. The substituted materials have shown poor wear along with low strength and low durability. When aluminum was substituted for the iron castings, special precautions had to be made to control the quantity of alloying ingredients, such as silicon, and the addition of lubricating substances, such as tin and lead, in order to achieve a reasonable degree of wearability (see U.S. Pat. No. 4,147,074).
In recent years vehicle manufacturers have turned to powder metal parts in the hope of reducing the processing costs, but have not been able to achieve high wear resistance in such parts. They have thus relegated powder metal to applications of low to moderate wear. For powdered parts to be utilized in critical applications, such as in valve seats, the powdered metal was made of a high alloy content material and the particles were agglomerated in conventional fashion (see U.S. Pat. No. 4,062,678 and 4,035,159). Each of these patents fail to exhibit superior surface wear resistance as demanded by critical engine applications today.
In the past, coatings have been applied to powdered metal parts to improve surface conditions, but in each instance the process involved multiple sintering steps with interposed coating steps, the coatings not being of a particularly low friction characteristic and typically were of a smearable type characterized by the group including copper, tin, zinc and lead (see U.S. Pat. No. 3,684,498 and 2,299,192). Techniques used in related arts to apply coatings to powdered metal parts have included plasma spraying of a melted alloy to obtain adherence by impact and high temperature bonding. Such spraying has usually been carried out on solid finished parts to obtain proper bonding. Plasma spraying suffers from high cost and the necessity for a post grinding operation. Another technique casts-in-place an equivalent to a coating about a desired part. Unfortunately, such cast-in-place coatings have not necessarily maintained successful adherence under service and have suffered from delamination. Moreover, such coatings undergo different shrinkage than the main body of the casting.